It is well known that chemical reactions may occur when certain plastic materials are exposed to sunlight or indoor fluorescent lighting due to the presence of ultraviolet light rays in these light sources. Such exposure causes visible discoloration, embrittlement, loss of tensile strength, or other undesirable changes in these plastic materials. Such occurrences are much more pronounced when the plastic is in the form of a thin coating or film rather than a heavier structure such as a bottle or tube which provide a greater bulk for the light rays to penetrate.
One well known method of inhibiting the harmful affects of ultraviolet light exposure is to incorporate opaque pigments into the plastic material. Although extremely effective in reducing degradation induced by ultraviolet light, these pigments render the article opaque to visible light, an unwanted situation where transparency of the article is important.
The preferred ultraviolet light inhibiting additives for normally transparent thermoplastic compositions are organic compounds which can be blended with the thermoplastic material and will inhibit the harmful effect of ultraviolet light exposure without interfering with transparency. A number of organic compounds are known to inhibit degradation of many plastic materials or resinous compositions but these are generally very selective and will not be compatible with a wide variety of plastic materials. For example, 2,4-dihydroxy benzophenone is known to be a useful ultraviolet light inhibitor for cellulose resins, epoxide resins, polyester resins and polystyrene but it is not recommended for use in acrylonitrile/butadiene/styrene polymers, polyethylene, polypropylene and vinyl polymers.
Other considerations in selecting an ultraviolet light inhibiting additive for a particular polymeric material or resin depend on economic factors and technical considerations which may not be related to ultraviolet light absorbents. The ultraviolet light inhibiting additive compound should have relatively high solubility in the polymeric material to which it is added and a low rate of loss from the material through exudation, volatilization, leaching or other means. The compound should have no chemical reactivity with the plastic material, other additives which may be present in the plastic material, or catalyst residues or other contaminants typically found in the plastic material. Preferably, the additive compound should have little or no color and provide little or no color to the plastic material to which it is added and such a condition should remain even upon exposure to heat or light. The additive compound should have low toxicity and should not induce skin reactions or other adverse effects when it is placed into contact with hands or other parts of the body. The additive compound should be easy to compound with the polymeric material and should also have the lowest possible cost consistent with desired performance for the composition.
In some instances some of these factors are unimportant, depending upon the intended use of the polymeric material with which the additive is blended. For example, in a formulation of polyethylene for ultimate use as a tarpaulin, color and toxicity are of minor importance, and high ultraviolet light absorbing efficiency and performance are of major importance. In the case of polystyrene light fixture diffusers, low initial color and freedom from discoloration upon exposure to light are prime considerations.
A wide variety of ultraviolet light absorbers is presently available to meet, in a limited degree, each of these requirements. A listing of such ultraviolet light absorbers may be found, for example, in the article by C. H. Fuchsman and F. R. Hansen commencing on page 436 of The Modern Plastics Encyclopedia published by McGraw Hill Inc., New York, New York (1972) and in the chart of ultraviolet light absorbing materials found on pages 488-489 of the same publication. Further information on such materials is provided in an article by R. F. Jackson commencing on page 264 of Modern Plastics Encyclopedia, published by McGraw Hill Inc., New York, New York (1974/1975).